Cobalt base alloy



United States Patent O U.S. Cl. 75-170 1 Claim ABSTRACT OF THE DISCLOSURE A high damping and high strength cobalt base alloy having between 60 and 75 percent cobalt, 20 to 39 percent iron and a total of molybdenum and tungsten of 0.5 to 4 percent.

This invention relates to cobalt base alloys and more more particularly to cobalt base alloys having high damping capacity as well as high strength, proper ductility and toughness.

In machine parts or elements, such as turbine blades,

for example, Where jarring, vibration, shock or sonic vibrations during service are likely to cause fatigue failure owing to the repeated stress, it is desirable to minimize the resulting stresses by attenuating the vibrations. This can be done effectively by making the machine parts or elements from a material with a high damping capacity, i.e. a material that has the ability to absorb vibration by internal friction, converting the mechanical energy 'into heat. This ability limits the development of excessively high vibration stresses in the material and thus increases its life under severe -conditions of vibration.

Heretofore, where high damping capacity materials have been required, 13% chromium type stainless steels, such as AISI 403 steel, and the high cobalt alloy known as NlVCO-lO have been employed. However, these alloys ordinarily have yield strengths of less than 80 kg./mm.2 at best, and while the yield strength of 13% chromium type steel can be improved by heat treatment, the steel thus treated shows a great loss in damping capacity.

The general objects of the present invention are to provide an alloy eliminating the diculties with prior types of alloys and having improved damping capacity while retaining excellent yield strength. Briefly, this result is attained by the provision of a cobalt base alloy comprising from 60% to 75% by weight cobalt, from 0.5% to 4% of either molybdenum or tungsten, or total content of the two elements, and not more than 0.06% of carbon, the balance being iron with the usual impurities such as phosphorus and sulphur, and deoxidizing agents such as manganese and silicon. These are kept to a minimum, so that the iron content can vary from a maximum of about 39% to a minimum of about 20%.

In the drawings,

FIGURE 1 is a diagram showing the relationship between Vickers hardness and yield strength and the damping capacity of an alloy made according to the present invention as compared with a similar curve showing the characteristics of a 13% chromium stainless steel; and

FIGURE 2 is a diagram showing the relationship between damping capacity and maximum bending stress in tuning type specimens in an alloy embodying the present invention.

The alloy forming the subject matter of the present invention is characterized in that it contains a larger percentage of cobalt than the usual cobalt base alloy, in combination with molybdenum or tungsten to give agehardening characteristics, with the balance consisting of iron and the usual impurities such as the residual amounts of carbon, phosphorus and sulphur, and manganese and silicon added for deoxidizing purposes. These elements contribute nothing toward the desired property of the alloys of the invention and preferably should be minimized.

Preferred alloys made according to the invention have compositions within the ranges given below:

Percent by wt. Cobalt Not less than 60% or more than Molybdenum, or alternatively tungsten or the total of both when the two alloy elements are addedin combination Not less than 0.5% or more than 4%.

Carbon Not more than 0.06%. Manganese Not more than 0.5%. Silicon Not more than 0.5%. Phosphorus Not more than 0.010%. Sulphur Not more than 0.010%. Iron Balance.

The percentage by weight of cobalt in the alloy should be within the range of about 60% to about 75% for the -reason that if the cobalt content is less than 60%, the

alloy is excessively brittle at room temperatures and is not suitable for practical service; if the cobalt content exceeds 75 a typical face-centered cubic crystal structure appears and the yield strength of the alloy is lowered.

Molybdenum or tungsten, or a combination ofthese two elements, are added to improve the age-hardening characteristic of the alloy. If the content of these two elements totals less than 0.5%, the alloy becomes dilcult to age harden While amounts greater than 4% result in alloys that are too hard for practical use and in which ductility and toughness are seriously reduced. Furthermore, if molybdenum or tungsten, or combinations of molybdenum and tungsten, are added in amounts greater than 4%, the alloy has increased hardness in the as-quenched state and face-centered cubic crystal structure also appears.

The carbon content preferably should be less than 0.06% because excessive carbon increases the hardnessof the alloy in the as-quenched state and lowers machinability.

Manganese and silicon should be below 0.5%. These additions are for the purpose of deoxidizing the alloy and serve no purpose in the finished alloy. Therefore, where oxygen can be removed by vacuum melting or another process, these elements are not necessary to the alloy.

Phosphorus and sulphur should be reduced to below 0.01% in accordance with common practice.

Depending primarily upon the amount of cobalt in the alloy, and depending to a lesser degree in the amount of the other alloying constituents, the iron, which makes up the balance of the alloy, varies from about 20% to about 39% by weight. The iron content assists in preserving the base metal of the alloy in body-centered cubic crystal structure. With the base metal in body-centered cubic crystal structure, the slip system dominates in dislocation with resultant increase in ductility and toughness of the all-oy.

The addition of a small amount of boron, while not essential, improves the properties of the alloy because boron helps to prevent loss of ductility and toughness by inhibiting the nucleation of precipitates on the grain boundaries during aging. Boron is added preferably at the rate of more than 0.003% and less than 0.05%.

Alloys made according to the present invention are preferably heat treated by quenching into water or oil from temperatures of between 950 C. and 1050 C. down to room temperature and then aging at temperatures 3 of from about 450 C. to 650 C. for periods of from one to one hundred hours.

Alloys made according to the present invention have hardness after quenching less than about 380 by Vickers hardness test and do not show excessive embrittlement even at elevated temperatures. The alloys, therefore, have good Weldability. Further, the alloys can be machined without diiiiculty by machining in the as-quenched state and then subsequently aging. The relatively low hardness in the as-quenched state gives good machinability. The temperatures required for aging are not excessively high, and therefore warping and distortion of machined parts during aging is minimized.

Table 1 below sets forth the chemical compositions and percentages by Weight of typical alloys embodying the invention. Table 2 below shows the typical properties of alloys having the compositions shown in Table l and subjected to the heat treatment given in Table 2. It will be noted that three tests were made of alloy A, these tests showing the result of different heat treatments. The test specimens were taken from mm. by 40 mm. square bars which were forged from 30 kg. ingots. The alloys were melted in a high frequency induction furnace.

composed of alloys made according to the present invention and heat treated to give the desired characteristics. Curve B of FIGURE 1 was made by testing in the sa-me manner several specimens of a 13% chromium. stainless steel alloy that had been heat treated to different yield strengths andhardnesses. The stainless steel alloy bears the designation SUS 37 B according to the Japanese IndustrialnStandard. AISI 403 steel gives approximately the same results as those shown by curve B.

FIGURE 2 shows the damping capacity (logarithmic decrement) of test specimens A-l, A-2 and A-3, "having the composition shown in Table 1 and heat treated as shown in Table 2, plotted against maximum bending stress in kg./mm.2 It will be noted that specimen A-l, which has a yield strength of 154.3 lig/mm.2 as shown in Table 2, has excellent damping capacity, although it `has the lea st damping capacity of the specimens shown 'on the chart, and that the damping'capacity of specimens A-2 and A-3, which were given heat treatment to lesser yield strengths, is rgreatly increased as compared to specimen A-l.

From the foregoing, it will be evident that the invention TABLE 1.-CHEMICAL COMPOSITIONS (PERCENT BY WEIGHT) Co Mo W B C Mn S1 P S Fe 71 23 2. 23 0 03 0.30 0.22 0.006 0. 003 Remainder.

. 0 003 0.03 0.31 0.21 0.006 0.004 Do. .04 0.23 0.24 0.007 0.007 Do. 0.02 0.32 0.31 0.005 0.005 D0. 0.01 0.22 0.23 0.008 0.006 Do. 0. 03 0.27 0.22 0. 007 0. 004 Do. G 71. 24 1.12 1.35 0.04 0.26 0. 25 0.006 0.005 D0.

TABLE 2.-MECHANIOAL PROPERTIES Yield Elongation strength Ultimate mm.

tensile gauge Reduction Impact Alloy Heat treatment,1 otset, strength, lengt of area, test 2 No. aging )rg/mm.2 kgJmxn.l percent percent value 1 All samples were oil quenched from 1,000 C. to room temperature and then aged as indicated in the table.

2Charpy impact test with 2 mm. V notch. Kg, meters/cm3.

It will be noted that the heat treatment imparts excellent tensile strength to the alloys, and that the alloys have good qualities with respect to elongation, drawing and Charpy impact tests.

The vibration damping characteristics of the alloys are shown in FIGURES 1 and 2 of the drawings. As shown in the curves constituting FIGURE 1 in which the maximum damping rate or capacity, expressed in terms of the logarithmic decrement, of the alloys is plotted on a logarithmic scale against the Vickers hardness and the yield strength of the alloys for an alloy heat treated to a given strength, the damping capacity of alloys according to the present invention greatly exceed the damping capacity of the previously used 13% chromium stainless steel, and in addition, the present alloys can be heat treated to much greater strength and hardness than the 13% chromium stainless steel.

The damping capacity was determined in accordance with test procedures similar to those described in an article entitled Turbine-Blade Vibration Due to Partial Admission by R. P. Kroon, in the Journal of Applied Mechanics, December 1940, pages A-161 to A-165. Curve A of FIGURE 1 was made by testing several specimens provides improved cobalt base alloys that maintain high damping capacity even when heat treated to substantial yield strength and hardness. It will also be evident that the alloys are readily machinable in the as-quenched state and that the heat treating procedures can be carried out without diiiiculty.

We claim:

1. A high damping and high strength cobalt base alloy consisting essentially of the following:

Percent by weight (References ou following page) 6 References Cited FOREIGN PATENTS UNITED STATES PATENTS 710,413 6/ 1954 Great Britain.

2,097,176 10/ 1937 de Gol er. 2,215,459 9/1940 Canel et a1. HYLAND BIZOT Pfmafy Exammef 2,974,037 3/ 1961 Thielemann. 5 G. K. WHTE, Assistant Examiner 3,046,108 7/ 1962 Eiselstein.

3,331,715 7/1967 Bulina et al 148--158 X 

